The present invention relates to a line-pressure control system for a V-belt type continuously variable transmission (CVT), which allows correction of the line pressure or source pressure for shift control of the CVT.
The V-belt type CVT comprises a primary pulley for receiving engine rotation, a secondary pulley coupled to wheels and having a V-groove aligned with that of the primary pulley, and a V-belt looped over the primary and secondary pulleys to engage in the V-grooves. The primary pulley, the secondary pulley, and the V-belt constitute a power train. In order to allow speed conversion of the power train, one of the flanges for defining the V-groove of each of the primary and secondary pulleys includes a stationary flange, and another includes a movable flange which can be displaced axially. The movable flanges are biased toward the stationary flanges by the primary-pulley pressure and the secondary-pulley pressure produced from the line pressure as source pressure, putting the V-belt in frictional engagement with the pulley flanges, thus allowing power transfer between the primary and secondary pulleys.
At the time of shifting, a shift actuator comprising typically a step motor is moved to an operated position, i.e. by a given step number, corresponding to a target shift ratio, producing between the primary-pulley pressure and the secondary-pulley pressure a differential pressure corresponding to the target shift ratio, changing the width of the V-grooves, thus achieving the target shift ratio. Using the Step-Ip characteristic shown in FIG. 10, a required step number Step of the step motor is looked up in accordance with a target shift ratio Ip corresponding to the driving conditions.
Since the line pressure serving as source pressure of the primary and secondary pressures adopts as a medium hydraulic oil out of an oil pump driven by the engine, the magnitude of the line pressure has a great effect on engine fuel consumption. Thus, it is commonly designed to control the line pressure controlled at a minimum value.
Due to determination of the line pressure, the possibility of deficient line pressure caused, e.g. by hardware variations cannot be removed completely. In that case, referring to FIG. 11 wherein the Step-Ip characteristic in FIG. 10 is shown by broken line, displacement occurs as shown by solid line in FIG. 11. Thus, even for achieving the same target shift ratio Ip, it is required to command the step motor by increasing the step number Step by an excess a, leading to delayed achievement of the target shift ratio Ip or impossible achievement thereof when the maximum-speed or highest shift ratio is required (hereafter refer to as “highest-shift-ratio unachieved state”).
Referring to FIG. 12, the highest-shift-ratio unachieved state is described with regard to when controlling the differential pressure between the primary-pulley pressure and the secondary-pulley pressure by operation of the primary-pulley pressure as is carried out widely. Referring to FIG. 12, an x-axis designates stroke amount or opening of a shift control valve operated by the step motor, and a y-axis designates hydraulic pressure. When having full line pressure as shown by solid line, the primary-pulley pressure produced from the line pressure as source pressure reaches a required primary-pulley pressure corresponding to the target shift ratio Ip (=highest shift ratio) at a stroke amount L1 as shown by solid line, allowing achievement of the highest shift ratio. On the other hand, when the line pressure is lower than the required primary-pulley pressure corresponding to the target shift ratio Ip (=highest shift ratio) as shown by broken line, the primary-pulley pressure produced from the line pressure as source pressure does not reach the required primary-pulley pressure as shown by broken line, leading to the highest-shift-ratio unachieved state.
Typically, the acting area of the primary-pulley pressure operated at the time of shifting is set roughly twice as large as that of the secondary-pulley pressure for convenience of control. In view of requirement of downsizing of the V-belt type CVT, it is a recent attempt to design the primary-pulley pressure acting area to be equal to the secondary-pulley pressure acting area. In that case, the secondary-pulley pressure is sensed by a sensor to be feedback controlled in accordance therewith, whereas the primary-pulley pressure is feedforward controlled through the shift control valve, rendering the above problem more serious.